Append example search path to cope with running in virtualenv (1.8.4) under Linux.
Added the update_sheet_activity function which updates sheets individually
Forcing update_activity in measure_response to ensure update regardless of topo.sim.time()
Updated imagen submodule reference
Merge pull request #517 from acwong/master

Append example search path to cope with running in virtualenv (1.8.4) under Linux.
Fix to ensure sheetviews are backwardly compatible with older snapshots
Merge remote-tracking branch 'upstream/master'

Conflicts:
	external/imagen
	topo/analysis/featureresponses.py